Brennan Linsley, Associated Press
People enter and leave the Arapahoe County District Court in Centennial, Colo., Monday April 13, 2015. The jury selection process in the trial of Aurora theater shooting suspect James Holmes entered its final stage Monday when attorneys began questioning prospective jurors as a large group. Holmes is charged with killing 12 people and wounding more than 50 in a crowded Aurora, Colorado movie theater in 2013.

CENTENNIAL, Colo. — A jury could be seated Tuesday in the Colorado theater shooting case, capping a nearly three-month selection process that experts say was among the largest and most complicated in U.S. history.

James Holmes is charged with killing 12 people and injuring 70 others in the July 2012 attack in suburban Denver. His attorneys don't dispute that he pulled the trigger but say he was in the grips of a psychotic episode when he slipped into the packed movie theater and opened fire.

Jurors will decide whether he was legally insane at the time. If they find him guilty, they must also decide whether he should be put to death or sentenced to life in prison without parole.

WHAT'S TAKEN SO LONG?

Some prospective jurors have asked the judge why it has taken nearly three years for the case to come to trial.

But Judge Carlos A. Samour Jr. has said that's not an unusual amount of time for a trial this complex. The death penalty and insanity plea introduced complicated and time-consuming legal requirements.

Experts say jury selection alone has been among the nation's largest and most complex. Court officials initially summoned 9,000 prospective jurors, who started filling out written questionnaires in January.

Hundreds were then asked to return for one-on-one sessions, where defense attorneys, prosecutors and the judge asked them about their views on the death penalty and mental illness.

Just 93 candidates returned for the final round of questioning Tuesday.

Once the jury is seated, the trial is expected to last another four or five months. Opening statements are set for April 27.

WHAT ABOUT OTHER LARGE TRIALS?

In the amount of time it has taken so far in Denver, federal jurors in Boston convicted marathon bomber Dzhokhar Tsarnaev.

That case was accelerated by Tsarnaev's admission that he participated in the April 2013 bombings and that his brother, Tamarlan, was the mastermind.

Tsarnaev's trial initially was expected to last three to four months, but it could end in about half that time. The penalty phase, when the same jury will decide whether to sentence Tsarnaev to death or life in prison, is scheduled to start April 21.

The Texas trial for the killer of a former Navy SEAL depicted in the movie "American Sniper" was complicated by publicity about the film. But jury selection moved quickly because it did not involve concerns about the large number of people affected by the crime.

In other recent mass killings, the suspect has killed himself, been killed by police or pleaded guilty before the case went to trial.

THE CHARGES

Holmes is charged with 165 counts, including numerous first-degree murder and attempted murder charges. He also faces an explosives charge, because he is accused of booby-trapping his apartment in a failed attempt to cause even more carnage.

The murder and attempted murder charges appear twice in court documents. On Monday, it took Samour nearly two hours to read each one, naming every victim, aloud in court.

THE ALTERNATES

In addition to 12 jurors, Samour hopes to have as many as 12 alternates because of the greater likelihood that jurors could have hardships during the lengthy trial.

But he said Monday there might be fewer alternates if too many people still in the jury pool are excused.

WHAT'S NEXT

The prosecution and defense each get 75 minutes to question prospective jurors as a large group Tuesday.

They will be able to ask the judge to dismiss people for such reasons as their inability to be fair and impartial, and any personal connections they have to the case.

Each side can dismiss up to 22 people without giving a cause.